Have you checked your backyard lately? You may be harboring dangerous aliens. No, these are not creatures from outer space. The aliens I’m referring to are plants – exotic plants. Also referred to as foreign, non-indigenous, introduced or nonnative, these invasive exotics are noxious weeds according to the following definition: A noxious weed is a plant alien to a geographical area whose presence threatens natural and agricultural ecosystems. Like their science fiction counterparts, these weeds are a major threat.

Don’t Be Fooled by Looks

At first glance, they seem harmless enough. Many have beautiful flowers, interesting leaves or unusual fruits. You were probably attracted to them in the first place because of these characteristics and because they were fast, easy-to-grow plants that required little care.

The trouble is most invasive weeds are attractive. They don’t look threatening at first. Incredibly, many are actually sold in nurseries as ornamentals. If they’d just stay put in the garden, the way other imported exotics do, they’d be welcome. Instead they escape and naturalize. Because they have been introduced into an environment in which they did not evolve and are thus free of the vast and complex array of natural controls present in their native lands (such as herbivores, parasites and pathogens that might limit their reproduction), they quickly get out of control. Invasiveness is usually characterized by rapid, robust growth that easily spreads over large areas. Exotic invasives often release toxins that poison our native plants. In their homelands, the other native plants had thousands, even millions of years to evolve immunities to their toxins. Our natives have not.

However, non-indigenous species are a very important part of our heritage and livelihood. Many are highly beneficial. Almost all U. S. crops and domesticated animals, many sport fish, numerous horticultural plants and biological control organisms have origins outside our country.

Nothing New

Invasive exotics are not new. They first came over on the Mayflower in cattle feed. By 1672, 22 weeds were documented in New England, including the ubiquitous dandelion, which is native to Northern Europe and Siberia. European settlers brought hundreds of plants from their homelands for food, medicinal, sentimental and ornamental purposes. Many of these plants have become naturalized to the point that we think of them as always being part of our landscape. The problem is that some of these naturalized plants have replaced our native plant species. Species native to North America are generally recognized as those occurring on the continent prior to European settlement.

More Than a Bother

Today, invasive weeds are everywhere – hundreds of them. They are not just a nuisance; they’re a real threat to the ecosystem and the economy. Cornell University Biologist David Pimentel calculated that the cost of battling these invaders for American industry, agriculture and health services is more than $122 billion a year.

In places where 150 species of native prairie grasses and flowers grew 10 years ago, there is now only one species of noxious weed. When this happens, we lose irreplaceable genetic plant material, as well as the bees, butterflies, birds and all the other creatures that depended on the diversity of the prairie for food and shelter. Where a multitude of flowers once bloomed in sequence over a seven-month period, there is now only one week of pretty color.

By displacing native plants, invasive exotics can change the entire structure of an ecosystem. Native insects, birds, mammals, etc., are dependent on native plants for food and shelter. Some species are actually restricted to feeding on only one or two plant species. The monarch butterfly caterpillar, for example, requires a host plant in the Asclepias genus (milkweed) during this stage of development.

Aliens to Watch for and Native Replacements

The following is a list of potentially dangerous plants when they escape from our gardens, along with suggested plants to use as substitutes.

The flower of purple loosestrife, shown here, belies the destructive nature of the plant itself.

The Japanese honeysuckle flower is both beautiful and sweet smelling but it can smother small trees and shrubs.

The highly invasive kudzu flower smells like grapes and can be used in jelly.

• Purple loosestrife (Lythrum salicaria) is a perennial and easily spotted because of its showy purple spikes. A native of Eurasia, it colonizes meadows, marshes, riverbanks and lakeshores. Extensive stands of it displace native vegetation, threaten rare and endangered plant species, and reduce food supplies and shelter for wildlife. The only creatures that eat this plant are European beetles. As a result, this “purple plague” has overrun wetlands in 42 states, from Maine to California, and put several species of amphibians and butterflies close to extinction. Some states have prohibited its sale. Replace it with blazing star (Liatris spp.), which has similar spiked, pink-purple flowers and is an important source of nectar for many native species of butterflies and other insects.

• Japanese honeysuckle (Lonicera japonica) is a woody vine with fragrant, white tubular flowers. Native to Asia, it was introduced here in 1806 as a landscape plant. Spreading rapidly via birds that eat the fruit, Japanese honeysuckle literally smothers small trees and shrubs, which collapse under its weight. Replace it with native trumpet honeysuckle (Lonicera sempervirens), which is an easy to manage relative that blooms heavily in spring, followed by scattered flowers in summer and fall.

• Kudzu (Pueraria lobata) is a climbing, semi-woody perennial vine that was widely planted to combat soil erosion in the early 1900s. It has beautiful sweet smelling flowers that are used to make jelly. Kudzu kills or damages other plants by smothering them under a solid blanket of leaves, encircling woody stems and tree trunks and breaking branches or uprooting entire trees and shrubs. Once established, kudzu grows at a rate of 1 foot per day! No wonder it has been called “the vine that ate the South.” Replace it with native vines, such as passionflower (Passiflora incarnata), native trumpet honeysuckle (Lonicera sempervirens) and native bittersweet (Celastrus scandens), which have attractive flowers and fruits and provide food for wildlife.

• Multiflora rose (Rosa multiflora) is a thorny perennial shrub that was introduced to the U.S. from Japan in 1886 as a rootstock for ornamental roses. In the 1930s, its use was advocated for erosion projects as a way to confine livestock and as a crash barrier in highway medians. The multiflora rose invades and damages pastures, crowds out vegetation, creates dense thickets, and even causes low crop yields on adjacent fields by competing for nutrients. Replace it with native roses, such as the Carolina rose (R. carolina) or the climbing R. setigera, which do not form extensive infestations.

• Tree-of-heaven or varnish tree (Ailanthus altissima) is a rapidly growing deciduous tree that can flourish in the most unfavorable conditions. Common nursery stock as early as the 1840s, its root system is aggressive enough to cause damage to sewers and building foundations. Each tree can produce as many as 325,000 seeds per year, which are easily dispersed by wind. Replace it with native trees and shrubs such as hickory (Carya spp.) and ash (Fraxinus spp.).

• Privet (Ligustrum spp.) is a perennial shrub that readily grows from seed or from root and stump sprouts. Commonly found in home landscapes, privet escapes cultivation by movement of seeds, which are eaten by wildlife, particularly birds. The fruit is not particularly good forage, but since every privet plant in the wild is depleting the resources for native wildlife, the birds are forced to eat the seeds and thus continue the cycle.

• Mimosa (Albizia julibrissin) is a deciduous leguminous tree that bears fragrant, bright feathery flowers from May to July, followed by legume pods in clusters that split open and reseed. Seeds may remain viable for many years. The mimosa tree was introduced from Asia in 1745 as an ornamental and potential forage. Replace it with native coralberry (Symphoricarpos orbiculatus).

The flower of the water hyacinth is one of the reasons many water gardeners are attracted to it but it is one of the most costly invasives to control.

• Water hyacinth (Eichhornia crassipes) is an aquatic herb and one of the most costly invasives to control. Native to South America, it forms a solid mat on the surface of water, crowding out native vegetation and forming dense shade that changes water temperature and kills submerged plants. People in the Gulf States consider it a contender for the title of “Worst Weed in the World.” With the popularity of water gardening and home ponds, gardeners should be aware of this potential problem.

• Chinese wisteria and Japanese wisteria (Wisteria sinensis and Wisteria floribunda) are exotic, showy, woody ornamental vines. Both plants were brought to the U.S. in the 1800s as ornamentals and have been grown extensively in the Southern states as decorative additions to porches and gazebos. Most infestations in natural areas are the result of escapes from landscape plantings. These vigorous vines impair and overtake native shrubs and trees through strangling or shading. Replace it with American wisteria (Wisteria frutescens), which is native to the Southeast U.S. and flowers June through August.

When gardeners, landscapers, and government agencies choose plants to grow, their choices can have widespread implications. Many invasive plants are spread by unsuspecting gardeners. Gardeners and the people who sell plants to them should become familiar with and watch for the known problem plants. When choosing species that are prone to “wander,” they facilitate the spread of those invaders to new areas. Instead, insist on species that are not invasive to help the environment and the economy.

“If dandelions were rare and fragile, people would knock themselves out to pay $14.95 a plant, raise them by hand in greenhouses, and form dandelion societies and all that. But they are everywhere and don’t need us and kind of do what they please. So we call them ‘weeds’ and murder them at every opportunity.” – Robert Fulghum, All I Really Need to Know I Learned in Kindergarten

Other Garden Favorites That Sometimes Act Aggressively

• Bouncing bet (Saponaria officinalis) is a perennial that was brought from England to be used in the milling industry for cleaning and softening cloth. From these waterway locations, it quickly escaped into the wild.

• Queen Anne’s lace (Daucus carota) is a biennial common to dry fields, ditches, and open areas. Introduced from Europe, it was used as a food source by the early settlers.

• Obedient plant (Physostegia virginiana) is a stiffly erect perennial that grows from creeping rhizomes. A native to Eastern North America, it occurs in swamps, streambeds, ditches, seepages, damp meadows, and prairies. Obedient plant can be an aggressive colonizer. To keep it “obedient,” plant it in drier soil and cut down on the fertilizer.

• Touch-me-nots (Impatiens balsamina) are an annual that easily reseeds. As the common name implies, the elastic seed capsules burst when touched, scattering seeds everywhere.

• Dame’s rocket (Hesperis matronalis) is a perennial planted as an ornamental, which quickly escapes cultivation because of its prolific seed set. Dame’s rocket has been distributed in wildflower seed mixes.

• Cornflower (Centaurea cyanus) is a hardy annual that will bloom abundantly in any kind of soil. A native to Europe, it has naturalized throughout North America. Often found along roadsides and in fields of corn, the stems are so tough, they have been called “hurt sickle” because of the way they dull and bend the edges of mowers.

• Chicory (Cichorium intybus) is a prolific perennial that grows in difficult places where nothing else will grow. The plant’s roots run to great depths and are very difficult to pull up. Some consider it a weedy nuisance, while others appreciate its beauty and culinary and medicinal virtues.

• Trumpet creeper (Campsis radicans) is a native perennial vine that is robust and aggressive if left unchecked. Although some consider this plant a weed, hummingbirds love it.

• Ox-eye daisy (Chrysanthemum leucanthemum) is a perennial found throughout North America in meadows, pastures and roadsides, and is often dispersed with wildflower seeds.

• Sunflower (Helianthus annus) is a North American native first used and cultivated by Native Americans. Of all the world’s seed crops, it is the only one domesticated in North America. Unlike most domesticated crops, the wild form of the cultivated sunflower is still found in abundance, growing as a weed. The wild form is highly branched with small heads and small seeds in contrast to the single stem and large seed head of domesticated sunflowers.

• Jimson weed (Datura stramonium) is a very common weed in fields and waste places almost everywhere in the U.S. Gardeners are attracted to the large, white funnel-shaped flowers that appear from May to September. The prickly seedpods burst open when ripe, scattering numerous poisonous, black kidney-shaped seeds.

• Star-of-Bethlehem (Ornithogalum umbellatum) is a perennial toxic to most grazing animals. Originally introduced to Indiana as a garden plant, it has now gone wild along roadsides, in fields and in some lawns in the South.

The views and opinions expressed in this article are those of the author alone and do not necessarily reflect the views or opinions of State-by-State Gardening, its parent company or affiliates. The author is solely responsible for all content. Our articles are only meant to educate and entertain our readers. We are not medical professionals and cannot recommend the ingestion or topical application of any herbal remedy, poultice, tea, etc. Please consult a medical professional before ingesting any plant.